The present invention relates to a digital modulation signal transmission apparatus of an orthogonal frequency division multiplex modulation (OFDM) scheme which uses a pilot signal for demodulating a modulated signal, and more particularly, to a digital modulation signal transmission apparatus which enables transmission of an auxiliary signal in addition to a signal modulated with main information codes and pilot signals, as well as to a signal display method for visualizing a received signal.
It should be first noted that, when used herein, the information code refers to an encoded signal which represents main information to be transmitted such as video information, audio information and other data information; the pilot signal refers to a reference signal which is utilized as the basis for the phase and amplitude of a received signal when it is demodulated; and the auxiliary signal refers to a signal other than the information code and the pilot signal, as will be described later in detail.
In recent years, in the field of radio devices, the OFDM scheme has been in the limelight as a modulation scheme robust against multipath fading. A large number of applied studies on the OFDM are now under progress in the fields of next-generation television broadcasting, FPU (Field Pickup Unit), radio LAN and so on in many countries including European countries and Japan.
Here, the OFDM scheme is an acronym of an orthogonal frequency division multiplexing modulation in which information codes are transmitted by using a plurality of carriers orthogonal to one another. The trends of developments in OFDM-based UHF-band terrestrial digital broadcasting and associated schemes are disclosed in detail in “The Journal of the Institute of Image Information and Television Engineers”, Vol. 52, No. 11, pp. 1539–1545 and pp. 1658–1665 (1998).
As an example of the prior art, the UHF-band terrestrial digital broadcasting system in Japan will be described below. It should be noted however that this scheme involves an extremely complicated configuration, so that the following description will be made on the system which is simplified to such an extent that is required for understanding the present invention.
Beginning with description on the structure of a carrier in this broadcasting system, as illustrated in FIG. 4, this system uses a total of approximately 1,400 carriers within a frequency band W which is divided into 13 segments such that information codes of up to three channels (three layers) can be simultaneously transmitted. In a case that the information codes for three channels are transmitted, for example, about 470 carriers are used for each channel.
In this event, the number of segments and a modulation method used in each layer can be freely selected from several modes as shown in the above-mentioned documents. Within such selectable modes, a mode in which all segments are modulated in accordance with the same synchronous modulation scheme such as 64QAM (Quadrature Amplitude Modulation) can be applied as it is to other transmitters such as FPU.
Now, referring to FIG. 5, a prior art OFDM system based on the synchronous modulation will be described below in greater detail for an example in which all segments are modulated in accordance with the same 64QAM scheme to transmit information codes on one layer. FIG. 5 is a diagram representing the structure of the carriers of segments which are modulated in accordance with the synchronous modulation scheme, and only shows a low end portion of the frequency band in FIG. 4.
In a mode which uses all segments for transmission of information codes on one layer, it may be thought that a similar structure is repeated over the entire band.
In FIG. 5, the horizontal direction represents the frequency; the vertical direction, the lapse of time; and squares “□” arrayed in the horizontal and vertical directions each represent one carrier. Thus, one column of carriers “□” arranged in the horizontal direction with in the whole frequency band represents one symbol which forms part of an OFDM signal.
Further, a carrier “□” with “SP” inscribed within the square represents the position of a carrier for a pilot signal which is used for reproducing a reference signal during demodulation, while a carrier without any inscription within the square represents the position of a carrier for a signal modulated in accordance with the 64QAM scheme. As can be seen in FIG. 5, since the pilot signals are scattered both in the frequency direction and the time direction, they are designated as “SP” (Scattered Pilot).
As shown in FIG. 6, a signal modulated in accordance with the 64QAM scheme is represented by any of 64 signal points indicated by broken line circles on a complex plane defined by an I-axis (real axis) and a Q-axis (imaginary axis) which are orthogonal to each other, wherein the respective signal points are corresponded to 6-bit codes which are different from one another. For example, a signal point b on the I-Q complex plane in FIG. 6 is corresponded to a code “000001”.
The modulation processing in accordance with the 64QAM scheme involves dividing a sequence of input information codes in units of six bits, assigning each of the divided 6-bit codes to any one of the 64 signal points on the I-Q complex plane. Each of the 6-bit codes is converted to a signal corresponding to the coordinate of I-Q complex plane representing a signal point indicated by a solid line circle “◯” in FIG. 6, and outputting the converted signal.
On the other hand, the transmission signal is affected by noise and other interference during a transmission process and distorted (its amplitude and phase have changed). For example, a signal point indicated by circle “◯” in FIG. 6 for a transmitted signal c, when received, has moved to a position c′ indicated by a cross “×” in FIG. 6.
The demodulation processing in accordance with the 64QAM scheme involves selecting the signal point closest to the signal point for the received signal represented by “×”, from 64QAM signal points indicated by broken line circles in FIG. 6, and outputting a 6-bit code corresponding to the selected signal point. For example, a received signal indicates a signal point c′ as shown in FIG. 6, a signal point c is selected.
Therefore, for the demodulation processing, the correct signal point position indicated by the broken line circle associated with the received signal must be reproduced and detected. The reproduction of the position only require to find, for example, the direction and magnitude of a reference signal vector which represents the correct position of a coordinate point “a” of a pilot signal as a standard of the signal space in FIG. 6. A solid line rhomb “⋄” superimposed on the position of the coordinate point a in FIG. 6 represents the position of the signal point for the pilot signal SP. In other words, the pilot signal SP represents the reference signal vector.
The directions and magnitudes of the reference signal vector and other signal vectors of a received signal have been affected by multipath and so on, which may occur on a transmission path between the transmission side and the reception side, causing the phase to rotate and the amplitude to change as well, as shown in FIG. 7. It is therefore necessary to reproduce the correct signal vectors on the reception side based on the received reference signal vector (pilot signal). Since the reference signal vector is required for each carrier, the reference signal vector must be determined for a carrier without the pilot signal SP as well, based on a nearby pilot signal SP.
Here, while the phase and magnitude of the reference signal vector change every time or from one carrier to another, as described above, the manner of changing is generally expressed by a smooth curve and has a remarkable correlation in the time direction and in the carrier direction (frequency direction).
For this reason, the reference signal vector for a modulated signal A of an arbitrary carrier of an arbitrary symbol in FIG. 5 can be readily found by interpolation of a plurality of sporadically transmitted SP signals. FIG. 5 shows exemplary positions of SP signals which facilitate efficient interpolation.
In recent years, a transmitter of a digital modulation scheme makes good use of the features of digital signals, and specifically inserts, other than a main signal which is modulated with information codes and a pilot signal, additional information such as control information representative of the type of a modulation method or an error correcting code used in transmission of main information codes, an audio signal or a signal for controlling of a pan head of a camera located at a transmission destination, and so on as auxiliary signals separate from the main information signal and pilot signal. The inserted auxiliary signals are transmitted together with other signals associated therewith.
Likewise, the UHF-band terrestrial digital broadcasting system in Japan defines a method of inserting a carrier for transmitting TMCC (control information: Transmission and Multiplexing Configuration Control) and a carrier for transmitting AC (Auxiliary Channel) as auxiliary signals within the carrier structure illustrated in FIG. 5 for transmission. In this event, as a modulation method for the auxiliary signals such as TMCC and AC, a DBPSK-based (Differential Binary Phase Shift Keying) transmission immune to noise and distorted waveform is generally used such that the information can be transmitted even in any severe conditions.
Then, the auxiliary signals modulated in accordance with the DBPSK scheme are set at positions on one of axes on the complex plane on which a signal point for a pilot signal is defined, for example, the I-axis (real axis) direction. Specifically, as shown in FIG. 6, a signal point AUX shown by “Δ” used for transmitting an auxiliary signal is set in the same direction and same magnitude as the signal point “a” shown by “⋄” used for transmitting the pilot signal with respect to the origin. Therefore, the signal point “a” for the pilot signal is superimposed on the signal point for the auxiliary signal on the I-axis. It should be noted however that FIG. 6 shows the signal point “a” for the pilot signal and the signal point AUX for the auxiliary signal displaced from each other for purposes of promoting the understanding of the description.
Therefore, two signal points “AUX” found on extreme left and right sides on the I-axis represent the positions at ends of a signal point which moves to the right and to the left on the I-axis, as a result of the modulation in accordance with the DBPSK scheme or BPSK scheme. The aforementioned auxiliary signal is modulated and transmitted as a position on the I-axis. The pilot signal is also modulated in accordance with the BPSK scheme.
Additionally saying for reference, the DVB-T (Digital Video Broadcasting-Terrestrial) system, which is a terrestrial digital television system in Europe, has substantially the same configuration as the terrestrial digital broadcasting system in Japan.
In the digital modulation scheme as described above, adjustments of a receiver are generally made for signal points for all demodulated carriers using a vector scope or an oscilloscope by displaying corresponding positions on the complex plane (constellation) shown in FIG. 6 in order and at a high speed. That is, an operator adjusts the receiver as he or she is monitoring dispersion and shifting of average position of the displayed signal points.
Particularly, since the pilot signal SP and auxiliary signal TMCC for transmitting information on a modulation method for use in demodulation play very important roles in demodulating main information signals which have been modulated in accordance with the 64QAM scheme, it is necessary to carefully examine their adjustment conditions and receiving conditions.
In this context, since the DBPSK scheme is not related directly to the synchronous modulation scheme for transmitting main information, DBPSK-based signal points need not be observed for adjusting signal processing circuits for the synchronous modulation scheme.
However, since the OFDM scheme involves a large number of carriers and a complicated signal structure, particular carriers are affected by extremely high noise in the transmission path. For this reason, displaying of signal points for signals modulated in accordance with the DBPSK scheme will facilitate analyses on causes of failures and adjustments of signal processing circuits.
Particularly, in an OFDM-based transmitter, this adjustment while viewing the constellation is an even more important and indispensable method since it serves as a powerful tool for adjusting the receiver. For this reason, the foregoing adjustment accompanied with a displayed constellation of signal points has been widely used from before in the OFDM-based receiver as well.
The prior art has a problem in that no careful attention is paid to a manner of displaying signal points on the constellation, so that sufficient adjustments cannot be made although signal points are displayed on the constellation at great pains. Specifically, as is apparent from FIG. 6, when the amplitude at a signal point “AUX” for an auxiliary signal modulated in accordance with the DBPSK scheme is superimposed on the amplitude at a signal point “a” for a pilot signal, these signals cannot be distinguished from each other on the display.
As a result, it is difficult to determine whether a signal point under observation is a signal point for an auxiliary signal or a moved signal point for a pilot signal due to its distortion or the like, thereby making the adjustment more difficult on the contrary.